Frequently transformers are provided in a circuit to provide isolation of two parts of the circuit and so safety requirements to ensure electrical isolation dictate the provision of certain minimum levels of insulation between the primary and secondary coils. For example, according to IEC 380 safety requirements of systems working at 130 to 250 VAC, both basic insulation and supplementary insulation are required in one case between a primary winding and a secondary winding. A basic insulation can be of any thickness which can withstand 1250 VAC and must have a creepage distance, that is to say the minimum surface distance over the insulation from a primary wire to a secondary wire, of at least 3 mm. A supplementary insulation should consist of two layers of insulation of any thickness which can withstand 2500 VAC or a minimum of 1 mm thick insulating material and a minimum of 4 mm creepage. In another case, a reinforced insulation can be used to replace both the basic and supplementary insulation. In this way the primary and secondary wires can be separated by a minimum of 2 mm thick reinforced insulation and a minimum creepage of 8 mm.
Generally therefore transformers have been made by winding a primary coil of enamelled wire on a bobbin whilst the secondary coil, also of enamelled wire, is wound on another bobbin which is then assembled concentrically with the bobbin of the primary coil. The minimum insulation requirement is met by the wall thickness of the bobbins between the two windings, whilst the minimum creepage distance is achieved by the design of the bobbins and control of the position of the windings on the bobbins.
In practice this limits the minimum radial distance between the primary and secondary coils to over 1 mm. However, the larger the spacing between the primary and secondary coils naturally the larger the flux leakage or loss in the transfer of energy from one to the other.
Also during each AC cycle applied to the primary coil, the energy initially has to overcome leakage inductance before any energy transfer occurs to make a current flow in the secondary coil. This wastes time during the cycle since, during the initial part of every cycle no energy transfer occurs, and the higher the frequency the greater and the more significant is this time loss and so the lower the efficiency of the transfer.
It is therefore an object of this invention to reduce the flux leakage from such transformers and so improve the efficiency of energy transfer of a transformer.